The hydrogel contains water as the medium, so that it is useful as a gel having high biocompatibility and is used in various fields such as applications for articles of daily use such as paper diapers, cosmetics and aromatics.
Examples of a related-art hydrogel include natural polymer gels such as agarose, and synthetic polymer gels in which between polymer chains is crosslinked through a chemical covalent bond such as an acrylamide gel.
Recently, functional gels in which various functions such as material retention capacities, an external stimulus responsive performance and a biodegradability in consideration of the environment are imparted to a hydrogel, are attracting attention, and there are performed attempts for developing various functions by introducing functional molecules into the natural or the synthetic polymer gels using a copolymerization reaction or the like.
Thus, for imparting new functions to a hydrogel, it is necessary to study the nanostructure and the surface structure of the gel in detail, however, in the above method for introducing functional molecules using a copolymerization reaction, there are various problems such as problems in which the introduction rate of functional groups is limited and a precise molecule design is difficult, a problem of the safety of materials that remain unreacted, and a problem in which the preparation of the gel is extremely complicated.
As opposed to such a related-art “top-down type” development of functional materials, attracting attention is a “bottom-up type” study for creating functional materials by which atoms or molecules which are the minimum units of the substance are assembled and in the resultant assembly which is a supramolecule, new functions are discovered.
Also in the field of the gel, the development of a novel gel formed from a non-covalent gel fiber (so-called “supramolecule polymer”) produced by self-assembly of a low molecular weight compound is progressed. This “self-assembly” refers to such a phenomenon that in a substance (molecule) group in a random state at first, molecules associate spontaneously by an intermolecular non-covalent interaction or the like under an appropriate external condition to grow to a macro functional assembly.
The novel gel is attracting attention, because the control of the macroscopic structure or function thereof is theoretically possible by controlling an intermolecular interaction or a weak non-covalent bond of a molecule assembly according to a molecule design of a monomer.
However, with respect to the way of controlling the intermolecular interaction or non-covalent bond between low molecular weight compounds, there is not yet found an apparent methodology. In addition, in the study of the non-covalent gel, the study of self-assembly utilizing a hydrogen bond in an organic solvent precedes because of a relative easiness of the gel formation, and self-assembled compounds (that is, hydrogelators or the like) in an aqueous solution have been found only accidentally.
Hydrogelators for forming a non-covalent gel which have been reported until now are broadly divided into the following three categories.
(1. Hydrogelators Having an Amphipathic Low Molecular Weight Molecule as the Skeleton Thereof)
This hydrogelator is created with an artificial lipid layer as a model and examples of the agent include surfactant-type gelling agents having a quaternary ammonium salt portion as a hydrophilic portion and having an alkyl long chain as a hydrophobic portion, and amphoteric surfactant-type gelling agents in which hydrophilic portions of two surfactant-type molecules are coupled.
As one example of the hydrogel formed by such gelling agents, there is disclosed a molecule organizational hydrogel formed by adding an anion having a molecular mass of 90 or more to a dispersion aqueous solution of a cationic amphipathic compound having a branched alkyl group in the hydrophobic portion (Patent Document 1).
(2. Hydrogelators Having a Skeleton in the Motif of Intravital Components)
Examples of this type of hydrogelators include gelling agents utilizing an association between molecule-assemblies through a peptide secondary structure skeleton (such as α-helix structure and β-sheet structure).
For example, there are disclosed a gelling agent having an α-helix structure (Non-patent Document 1) and a gelling agent having a n-sheet structure (Non-patent Document 2).
(3. Hydrogelators Having a Semi-Artificial Low Molecular Weight Molecule as the Skeleton Thereof)
This type of hydrogelators is composed of a combination of intravital components (hydrophilic portion) such as DNA bases, peptide chains and sugar chains and alkyl chains (hydrophobic portion) and the like, and can be referred to as a gelling agent combining characteristics of the above two types of gelling agents. Here, the DNA base, the peptide chain and the sugar chain assume not only a role of enhancing the hydrophilicity, but also a role of imparting an intermolecular interaction such as a hydrogen bond.
For example, there are disclosed a hydrogelator containing a glycoside amino acid derivative having a sugar structure site having an N-acetylated glycoside structure of a monosaccharide or disaccharide (Patent Document 2) and disclosed a fine hollow fiber formed having a self-assembling property from a peptide lipid represented by General Formula: RCO(NHCH2CO)mOH and a transition metal (Patent Document 3).
In addition, it is disclosed that an amphipathic peptide having a structure of (hydrophobic portion-cysteine residue (forming a disulfide bond during the network formation)-glycerin residue (imparting flexibility)-phosphorylated serin residue-cell adhesive peptide) forms a β-sheet type fiber network with the hydrophobic portion as a nucleus (Non-patent Document 3).
In addition, there is also disclosed a case where a sugar lipid-type supramolecule hydrogel was produced using a chemical library (Non-patent Document 4).    [Patent Document 1]    Japanese Patent Application Publication No. JP-A-2002-085957    [Patent Document 2]    Japanese Patent Application Publication No. JP-A-2003-327949    [Patent Document 3]    Japanese Patent Application Publication No. JP-A-2004-250797    [Non-patent Document 1]    W. A. Pekata et al., SCIENCE, vol. 281, 389 (1998)    [Non-patent Document 2]    A. Aggeli et al., Angew. Chem. Int. Ed., vol. 42, 5603-5606 (2003)    [Non-patent Document 3]    Jeffrey D. Hartgerink, Elia Beniash, Samuel I. Stupp, SCIENCE, vol. 294, 1684-1688 (2001)    [Non-patent Document 4]    Shinji Matsumoto, Itaru Hamachi, Doj in News, No. 118, 1-16 (2006)    [Non-patent Document 5]    Kjeld J. C. Van Bommel et al., Angew. Chem. Int. Ed., vol. 43, 1663-1667 (2004)